Transcriptional Regulations on the Low-Temperature-Induced Floral Transition in an Orchidaceae Species, Dendrobium nobile: An Expressed Sequence Tags Analysis

Hindawi Publishing Corporation Comparative and Functional Genomics Volume 2012, Article ID 757801, 14 pages doi:10.1155/2012/757801 Research Article Transcriptional Regulations on the Low-Temperature-Induced Floral Transition in an Orchidaceae Species, Dendrobium nobile : An Expressed Sequence Tags Analysis Shan Liang,1 Qing-Sheng Ye,1 Rui-Hong Li,1 Jia-Yi Leng,1 Mei-Ru Li,2 Xiao-Jing Wang,1 and Hong-Qing Li1 1 Guangdong Provincial Key Laboratory of Biotechnology for Plant Development, School of Life Science, South China Normal University, Guangzhou 510631, China 2 Key Laboratory of Plant Resources Conservation and Sustainable Utilization, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou 510650, China Correspondence should be addressed to Xiao-Jing Wang, and Hong-Qing Li, Received 30 July 2011; Revised 21 December 2011; Accepted 8 January 2012 Academic Editor: G. Pesole Copyright © 2012 Shan Liang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Vernalization-induced flowering is a cold-relevant adaptation in many species, but little is known about the genetic basis behind in Orchidaceae species. Here, we reported a collection of 15017 expressed sequence tags (ESTs) from the vernalized axillary buds of an Orchidaceae species, Dendrobium nobile, which were assembled for 9616 unique gene clusters. Functional enrichment analysis showed that genes in relation to the responses to stresses, especially in the form of low temperatures, and those involving in protein biosynthesis and chromatin assembly were significantly overrepresented during 40 days of vernalization. Additionally, a total of 59 putative flowering-relevant genes were recognized, including those homologous to known key players in vernalization pathways in temperate cereals or Arabidopsis, such as cereal VRN1, FT/VRN3, and Arabidopsis AGL19. Results from this study suggest that the networks regulating vernalization-induced floral transition are conserved, but just in a part, in D. nobile, temperate cereals, and Arabidopsis. 1. Introduction Transition from the vegetative phase to the flowering phase is crucial to both development and reproduction in plants. Besides endogenous signals, this process is also affected by external cues such as day length and temperature. Vernalization, an exposure to low temperature extending over a period differing from species to species, is an adaptive nature to ensure some plants survival in harsh winters and flowers under a favourable condition in spring. In the dicot model Arabidopsis, vernalization-regulated flowering is mediated by both FLC-dependant and -independent pathways [1, 2], in which the gene VIN3 (which interacts with VRN2), a key upstream component, is probably activated by exposure to low temperature and subsequently leads to changes in histone methylation of downstream gene regions [3]. For example, the expression of FLC is suppressed by vernalization through enrichment of H3K27m3 on the chromatin [3], which consequently releases FT and SOC1 from inhibition by FLC to promote the transition to flowering. This FLC-dependant pathway is regulated by both temperature and day length [4]. AGL19, a close relative of SOC1, is believed to mediate an FLC-independent pathway that activates flowering under vernalization in Arabidopsis [1]. This process is associated with a cold-induced decrease of H3K27m3 on AGL19 locus and probably also with the loss of function of CLF and MSI1 or the involved complex [5]. Ectopically expression of Arabidopsis AGL19 leads to only mild abnormalities, suggesting that AGL19 has a limited role in flowering control in Arabidopsis [1]. In addition to FLC and AGL19, MAF2 and AGL24 are also involved in the floral transition in Arabidopsis [6, 7]. At present, FLC is thought to be central to the control of flowering in Arabidopsis and probably also in most other dicots [1, 2]. 2 Monocots, however, are likely to be different: no monocot orthologs of Arabidopsis FLC have been found so far. In temperate cereals such as wheat and barley, responses to vernalization are mediated by VRN1, VRN2, and VRN3 [2]. Cereal VRN1 has two roles, namely, (a) to transmit the cold signal and thereby induce VRN3, the ortholog of AtFT in cereals [11] and (b) to act as a floral meristem identify which is activated by FT/VRN3 [12]. Thus, VRN1 and FT/VRN3 form a positive feedback loop and regulate each other under vernalization. VRN1 may also have a role in storing the memory of vernalization through methylation of Histone 3 [13], which resembles the epigenetic property of Arabidopsis FLC. However, the exact mechanism of this process remains unclear so far. Cereal VRN2 acts as a flowering repressor that is regulated by both low temperatures and day length [14]. Cold can lower the expression of VRN2 [15], thereby releasing FT/VRN3 from repression [16]. VRN2 may be inhibited by VRN1 under vernalization [9, 10, 14]. To summarize, vernalization-induced regulation of flowering in monocots is evolutionarily divergent from that in dicots, although the networks are similar to some extent [2]. Orchidaceae is the largest family in the plant kingdom and is considered particularly speciational, indicating that Orchidaceae species are well adapted to the surrounding environments. Dendrobium is a vast genus consisting of more than a thousand species that are native to South Asia, Australia, New Zealand, and Oceania [17]. Species of this genus differ in the extent of dependence on vernalization. For instance, Dendrobium phalaenopsis can flower at high temperatures whereas Dendrobium nobile requires vernalization and flowers only after a spell of exposure to relatively low temperatures [18]. D. nobile is used as a herbal medicine and is popular as a potted ornament prized by growers and hobbyists for its beautiful flowers. Its juvenile period lasts for at least 3–5 years, which makes it difficult to meet the commercial demands. To accelerate flowering, potted D. nobile plants are always kept at low temperatures in a temperature-controlled greenhouse or grown in naturally cool environments such as in high mountains. In the laboratory, low-temperature treatment has been adopted for shortening the juvenile phase and promoting flowering [19]. Plants exposed to a constant temperature of 13◦ C flowered early whereas those at 18◦ C remained vegetative and did not flower [18, 20]. To summarize, vernalization can promote flowering in this species. However, the genetic basis of low-temperature-induced flowering in D. nobile remains poorly understood. Although a number of genes have been identified in orchids [21, 22], and several datasets of expressed sequence tags (ESTs) from flowering buds of Orchidaceae species have been deposited in GenBank, only a few of these genes have been characterized. Examples come (...truncated)